Bone constraint to 2 other bones
You can animate the influence of both constraints simultaneously. And don't move the sword if it has already set it's offset to parents. Just place your hand to the sword's handle, keyframe sword's 'hand' constraint influence at 0.0, and its 'sheath' constraint influence at 1.0, and then at the next frame, flip the influence, and keyframe them again ('hand' becomes 1.0, and 'sheath' becomes 0.0). You can use the gradual influence switch (not the next frame, but a few frames delay).
If you have to rotate the sword during the battle, all you need to return it to the sheath is to restore it's original rotation first (duplicating keyframe in the dopesheet or copypaste pose for that bone at different frames).
I don't know how does your sheath work, and as you say, the sword jumps into the hand. See attached blend, how I understood it.